(1) Field of the Invention
The present invention relates to a fibre amplifier, and more particularly to an erbium doped fibre amplifier (EDPA).
(2) Description of the Art
Optical fibre based networks are known and have been implemented in many forms using both single mode and multi-mode optical fibres. Multi-mode optical fibre based systems typically have a limited bandwidth due to modal dispersion effects within the fibre. Although modal dispersion effects can be reduced or eliminated by using parabolically graded index multimode fibres and the like, such fibres are difficult and expensive to manufacture. Multi-mode optical fibres are thus typically only used in low speed network applications.
Modal dispersion effects are, by definition, insignificant in single mode optical fibres and such fibres are thus used in high bandwidth data transmission systems. However, the small core diameter (typically less than 10 μm) that is required to maintain single mode propagation limits the optical power that can be coupled into the fibre. Optical amplifiers are thus often necessary when data is transmitted through long lengths of single mode optical fibre.
One known type of optical amplifier suitable for use in single mode optical fibre systems are so-called erbium doped fibre amplifiers (EDPAs). A typical EDFA comprises a single mode optical fibre having a core region formed from glass that is doped with erbium. A signal beam (e.g. of wavelength around 1550 nm) and a pump beam (e.g. of wavelength 980 nm or 1480 nm) are coupled into the doped optical fibre. The pump beam causes amplification of the signal beam within the erbium doped optical fibre.
The relatively low optical gain of erbium doped glass means that many meters (e.g. tens or hundreds of meters) of single mode optical fibre is required in a typical EDFA. Known EDFAs can thus be physically large and quite costly. Furthermore, the small core diameter places a limit on the maximum optical power output of known EDFAs due to non-linear effects within the fibre and radiation induced damage to the fibre core and/or to the end facet of the fibre.
U.S. Pat. No. 6,611,372 describes an optical amplifier arrangement in which a single piece of phosphate glass optical fibre is held in a v-groove channel in a substrate. The optical fibre described in U.S. Pat. No. 6,611,372 is typically less than 20 cm long and has a core diameter in the range of 50-100 μm.